Components used in gas turbine engines are commonly made of high temperature oxidation and corrosion resistant superalloys. Nonetheless, coatings are often applied to the surface of such alloys, to protect them from environments beyond their inherent capability, or to impart specialized surface properties.
Nickel aluminide coatings are used for such purposes. They are predominantly nickel, with small quantities (.about.5%) of aluminum. When applied as plasma sprayed coatings, the nickel aluminide materials can provide a surface which is oxidation and corrosion resistant, but which has more abradability than is obtainable in a conventional cast or forged superalloy. Thus, these types of coatings are utilized where there is a need for tight clearance, but the possibility of metal to metal contact exists. As a particular example, nickel aluminide coatings are used on the outer airseals in low pressure turbine parts of a gas turbine engine. The outer airseal pieces are made typically of a nickel superalloy, and form a circumferential ring which encircles turbine blades mounted in a spinning disc.
With use of a gas turbine engine, there is degradation of the nickel aluminide coatings, as occurs ordinarily with use of various components. Periodically, a gas turbine engine will be overhauled, and at this time it is desirable to refurbish the outer airseals. Because of the corrosion and oxidation products which form on the outer airseal surfaces, and to provide a good bond for new coating material, it is necessary to entirely remove the old coating. Using mechanical means, such as severe abrasive blasting or machining, risks removing parts of the base metal airseal and can be too costly.
Chemical removal of coatings using stripping solutions is an obvious choice. However, difficulty arises when there is a similarity in composition between the coating and the substrate base metal, as there is for nickel aluminide coated airseals. A chemical solution which attacks the coating may also attack the base metal of the component, producing either gross removal of substrate material (which is undesirable in the same sense as mis-machining is) or preferential corrosion attack and weakening of the substrate. Solutions which are weak, so that they do not attack the base metal, will be ineffective in removing the coatings within a commercially feasible time. Among the commercial stripping materials which have been found to be not satisfactory are conventional sodium cyanide aqueous solutions and proprietary alkaline non-cyanide salt solutions, such as Metex Strip Aid Salts (McDermid Corp., Waterbury, CT) and Enstrip-S (Enthone Corp., New Haven, CT), solutions which are well known in the prior art to be generally useful for stripping. The attack on nickel aluminide is unduly slow or non-existent, and removal is not possible even after hundreds of hours of immersion at room temperature.
There are some co-pending U.S. patent applications which have the applicants Fishter and Lada as common inventors, and which relate to chemical milling: Application Ser. No. 338,739, Selective Chemical Milling of Recast Surfaces, filed Jan. 11, 1982, relates to the removal of the recast layer resulting from electric discharge machining, as opposed to the removal of the base metal. Application Ser. No. 192,667, filed Oct. 1, 1980 now U.S. Pat. No. 4,353,780, Chemical Milling of High Tungsten Content Superalloys, relates to solutions which are suitable for chemical milling alloys which have relatively high tungsten content, compared to superalloys in general. U.S. Pat. No. 4,339,282 Method and Composition for Removing Aluminide Coatings from Nickel Superalloys, discloses a solution having nitric and hydrochloric acids, suitable for removing pack cementation (diffusion type) aluminide coatings from cast superalloys. Such coatings usually have 20-30 weight percent aluminum, plus silicon, in addition to the other elements which comprise the substrate, and are distinguishable from the low aluminum coatings to which the present invention is pertinent. Sizer et al. in Australian Patent AU-B 10761/76 discloses another method for removing diffused aluminum coatings from high Cr content superalloys by immersion in an aqueous bath of 5-40 volume percent nitric acid. Vigorous agitation is needed to prevent localized pitting of the substrate.
In U.S. Pat. No. 4,274,908 to Fishter et al., a chemical milling solution usable for removing gold-nickel type brazed material from superalloy articles is disclosed; the solution contains nitric acid, sulfate ions, chloride ions, and metal ions.
The foregoing applications and patents indicate that for each situation there appears to be a critical combination of chemicals which obtains the desired expeditious removal of unwanted material, but avoids deleterious attack of the substrate which is left after processing.